Skin aging is a progressive phenomenon, occurs over time and can be affected by lifestyle factors, such as alcohol consumption, tobacco use and sun exposure. Aging of the facial skin can be characterized by atrophy, slackening, and fattening. Atrophy corresponds to a massive reduction of the thickness of skin tissue. Slackening of the subcutaneous tissues may lead to an excess of skin and ptosis can and lead to the appearance of drooping cheeks and eye lids. Fattening refers to an increase in excess weight by swelling of the bottom of the face and neck. These changes are typically associated with dryness, loss of elasticity, and rough texture.
Hyaluronan, also known as hyaluronic acid (HA) is distributed widely throughout the human body in connective and epithelial tissues and abundant in the different layers of the skin, where it has multiple functions such as, e.g., to ensure good hydration, to assist in the organization of the extracellular matrix, to act as a filler material, and to participate in tissue repair mechanisms. However, with age, the quantity of HA, collagen, elastin, and other matrix polymers present in the skin decreases. For example, repeated exposed to ultra violet light, e.g., from the sun, causes dermal cells to both decrease their production of HA as well as increase the rate of their degradation. This HA loss can result in various skin conditions such as, e.g., imperfects, defects, diseases and/or disorders, and the like. For instance, there is a strong correlation between the water content in the skin and levels of HA in the dermal tissue. As skin ages, the amount and quality of HA in the skin is reduced. These changes lead to drying and wrinkling of the skin.
Dermal fillers are useful in treating soft tissue conditions and in other skin therapies because the fillers can replace lost endogenous matrix polymers, or enhance/facilitate the function of existing matrix polymers, in order to treat these skin conditions. In the past, such compositions have been used in cosmetic applications to fill wrinkles, lines, folds, scars, and to enhance dermal tissue, such as, e.g., to plump thin lips, or fill-in sunken eyes or shallow cheeks. One common matrix polymer used in dermal filler compositions is HA. Because HA is natural to the human body, it is a generally well tolerated and a fairly low risk treatment for a wide variety of skin conditions. Unfortunately, some HA compositions are less stable to sterilization, such as heat sterilization, than may be desired.